Split components into mono repo

client/, core/, and server/ should be independent without any cross dependencies.

Steps:

1. Create shared/ directory for code shared between server/ & client/
2. Move all non-core related code to the shared/ directory
3. Remove core/ dependencies on client/
4. Move public core api into engine_api/
5. Move all core private code to engine/
6. Ensure client & server only have dependency on engine_schemas/